Many pacemakers and ICDs include one or more leads mounted in the atria for directly sensing atrial events, particularly P-waves. Reliable P-wave detection is required so as to determine the atrial rate for detecting atrial fibrillation and for controlling atrial pacing functions, such as dynamic atrial overdrive (DAO) pacing. A long-standing problem with atrial sensing is that electrical signals generated in the ventricles (R-waves) often appear within the signals sensed within the atria and can be misinterpreted as P-waves. Electrical signals that originate in a different chamber of the heart from where the signals are sensed are referred to as far field signals. Electrical signals that originate in the chamber of the heart in which they are sensed are referred to as near field signals. Hence, the general goal is to discriminate near field signals from far field signals.
Since the ventricles are considerably more massive than the atria, depolarization of ventricles generates R-waves having magnitudes far greater than P-waves. Even when sensed with leads mounted within the atria, the far field R-waves often appear with at least the same magnitude as the near field P-waves, making it quite difficult to reliably detect only P-waves. Note that, strictly speaking, P-waves and R-waves are features of a surface electrocardiogram (EKG). Herein, the terms P-wave and R-wave are also used to refer to the corresponding internal electrical signal component, i.e. the corresponding component of an intracardiac electrogram (IEGM).
In dual chamber modes, far field R-wave sensing is normally thought of as an R-wave followed by a P-wave. A sufficiently long Post-Ventricular Atrial Refractory Period (PVARP) is used to prevent far field sensing when the sensing order is R-P, i.e. the R-wave is sensed before the P-wave. However, a second case of far field sensing exists in which the far field R-wave is sensed in the atrial channel before the associated R-wave is sensed in the ventricular channel. Traditional PVARP is not helpful in this case since the far field R-wave appears on the atrial channel signal before the PVARP begins.
The second type of far field sensing (i.e. the P-R type) is illustrated in FIG. 1, which shows atrial and ventricular sense amplifier signals 2 and 4, respectively, along with a corresponding surface EKG cardiac signal 6. As can be seen, an R-wave 7 appears as a far field signal in the atrial sense amplifier signals (though reversed in polarity in this particular example.) Far field sensing of the P-R type is often caused by the relationship between the atrial and ventricular sensing thresholds with respect to the sense waveforms. P-R type far field sensing is also dependent to lead positioning, as well as on the frequency response of any band-pass filters provided on the sensing channel. In particular, a narrow band-pass filter can cause ringing and peak detections away from the actual signal. Such filters are employed to limit sensed events to only P/R wave morphologies.
FIG. 2 shows a detailed example of far field R-wave sensing. Both atrial and ventricular channel signals 8 and 9, respectively, respond to a physiological ventricular depolarization. However, because of ringing and sensing threshold settings, the atrial channel 8 senses the far field R-wave before the R-wave is sensed in ventricular channel 9. In particular, the atrial channel senses the R-wave when its signal magnitude exceeds the atrial channel sensing threshold, which occurs before the ventricular channel senses the R-wave (when its signal magnitude eventually exceeds the ventricular channel sensing threshold.) The far field R-wave that is sensed on an atrial channel signal is misinterpreted as a P-wave and thereby resets timing intervals. A device with no protection against far field sensing of the P-R type (1) may be susceptible to large number of Auto Mode Switches (AMS), (2) may be less capable of detecting and responding to atrial arrhythmias, (3) may exhibit high inappropriate atrial rate Stored Electrograms, and (4) may record skewed diagnostics. In particular, the device may count both the true P-wave and the far field R-wave on the atrial channel for the purposes of atrial rate calculation, likely resulting in the calculated atrial rate being twice the actual atrial rate. For ICDs configured to deliver a high energy cardioversion shock to terminate atrial fibrillation, the erroneous calculation of the atrial rate may result in a painful cardioversion shock being delivered even though none is required.
P-R type far field sensing problems were addressed in U.S. Pat. No. 6,516,225 to Florio, entitled “System and Method for Distinguishing Electrical Events Originating in the Atria from Far field Electrical Events Originating in the Ventricles as Detected by an Implantable Medical Device.” Using techniques described therein, far field R-waves in the atria are distinguished from true P-waves using both a post ventricular atrial blanking (PVAB) interval and a separate pre-ventricular blanking interval (pre-VAB) interval. Upon detection of a P-wave in an atrial channel signal, the device begins tracking a pre-VAB interval. If an R-wave is then detected in a ventricular channel signal during the pre-VAB interval, the P-wave is rejected as being a far field R-wave. A PVAB interval may also be employed to filter out any P-waves detected in the atria immediately following detection of an R-wave in the ventricles. In another example, far field R-waves are distinguished from true P-waves using template matching. P-waves detected in the atria are compared against a template representative of true P-waves. If the P-wave substantially matches the template, the P-wave is deemed to be a true P-wave; otherwise the P-wave is rejected as being a far field R-wave or other anomalous electrical event.
Although the technique of Florio is effective, room for improvement remains. In particular, the technique of Florio is primarily a microprocessor-based technique, i.e. the algorithms performed to discriminate far field and near field atrial signals are implemented in software using a microprocessor. Although the techniques successfully detect and reject far field R-waves from the atrial channel, the techniques are relatively costly in terms of software complexity and microprocessor duty cycle time. With ever greater burdens placed on device microprocessors to detect and respond to a wide range of arrhythmias or other medical conditions, it would be desirable to instead provide hardware-based techniques for discriminating far field and near field atrial signals.
One hardware-based technique is described in U.S. patent application Ser. No. 10/430,039 of Kroll et al., filed May 3, 2003, entitled “System and Method for Rejecting Far field Signals using an Implantable Cardiac Stimulation Device.” A Boolean logic circuit is described therein for filtering far field electrical cardiac signals from near field signals wherein atrial tip and ring signals are sensed using unipolar electrodes. Any timing differences between corresponding events within the signals are detected. Then, far field signals are filtered from the tip and ring signals based on the detected timing differences, such that substantially only near field atrial signals remain. The circuit exploits the fact that near field atrial signals are sensed when a conduction wave passes by the atrial electrodes. In contrast, far field signals from the ventricles propagate to the atrium at near the speed of light. Hence, any significant timing difference between corresponding events appearing in the atrial signals is indicative of a near field event, whereas the lack of a significant timing difference is indicative of a far field event.
It would be desirable to provide alternative hardware-based techniques for discriminating far field and near field atrial signals.